9to5Mac's sources say that Siri won't need to hog the entire screen of the iPad (like it does on the iPhone), instead, it'll act more like how Notifications Center works on the iPad. Siri's interface is supposed to "built into the iPad as a small window on top of whatever interface the user is currently interacting with. The Siri window with rounded corners sits at the bottom center of the display". As Notifications Center is a drop down banner on the iPad, Siri on the iPad is expected be a drag up sliding screen. You can trigger Siri by holding the home button, just like on the iPhone 4S.